WLAN systems, such as those meeting the 802.11a/b/g standard, are used predominantly in PC applications. An HF filter that passes the desired frequency range and has sufficient suppression in the stop band is needed in both the receiver and the transmitter in these systems. In PC applications, however, it is not generally necessary to achieve a high level of suppression in the stop band.
There is, however, increasing interest in creating cross-system technologies, in particular combining WLAN technology and mobile radio, in order to use VOIP (Voice Over Internet Protocol) and other data transfer functions via cell phone, for example. In the case of integrating WLAN functions in a cellular radio environment for cell phones, a high level of suppression of the mobile radio frequencies is required in order to allow stable coexistence of the WLAN and the mobile radio systems.
Initial attempts to produce WLAN modules integrated in mobile radio systems were built from discrete components, and therefore required a relatively large module surface area.
For attempts using HF filters built using LTCC multilayer technology, there were problems integrating the filters into small, ceramic front-end modules. Discrete filters built based on LTCC technology, in contrast, are typically not compatible with the manufacturing process for LTCC modules. The integration of HF filters in LTCC substrates for front-end modules also causes problems, because the HF filter integrated in the substrate becomes unstable due to the high level of coupling between the LTCC material of the module and the power amplifier.
It is further possible to develop such modules, suitable for WLAN and mobile radio, on the basis of laminate or LTCC technology, and to use discrete components based on LTCC, SAW, or FBAR technology for the corresponding filters. Using these components, good module properties and reliable manufacture can be expected. The disadvantage, however, is the size of such modules, which require relative large module surface area.